Resolution:
First confirm that all Display Network and Email configuration settings are configured properly.
Test if you can ping the display and also test you can ping the DNS service and backup DNS server, if you can then try to reboot the display to save the changes made to the email settings.

To reboot the display locate the calibration/ reboot display feature under the service settings screen or use the pin hole reset button top of the display. Please note that rebooting the display will not affect the UPS operation mode, it is safe to do while load is on the UPS.

Display reboot should allow the emails settings to save and now the “Test Email” functionality should work.